1. Field of the Invention
The invention involves a photographic silver halide material for preparing silver images, using a sensitizing dye that provides low stain at high sensitivity.
2. Description of the Related Art
The use of polymethine or cyanine dyes to spectrally sensitize silver halide emulsions has long been known. Numerous types of these dyes are described in relevant handbooks, for example, T. H. James "The Theory of the Photographic Process", 4.sup.th edition, pages 194 ff, Mac Millan Publishing Co., New York, 1977. However, not all dyes of this class are equally suitable for use in silver halide emulsions.
Photographic silver halide materials, for example, for preparing black and white silver images for prepress operations, have a series of special requirements:
They must have the highest possible spectral sensitivity. PA1 They must be rapidly processible. PA1 The images should not show, as a result of rapid processing, troublesome stain from sensitizing dyes residues.
It has become customary in recent years to shorten the processing cycle for exposed silver halide films, which usually comprises treatment with at least three different aqueous baths for developing, fixing, and washing, by increasing the developer bath temperature and by using concentrated developer and fixing bath solutions. However, the sensitizing dyes are often not adequately removed from the layer during processing, and a troublesome residual stain remains. This can show, for example, in black and white images, as unsatisfactorily high minimum density relative to light of a certain wavelength, and in color images, as color adulteration.
It is indeed possible, within certain limits, to increase sensitization, that is, to increase spectral sensitivity, by increasing the quantity of sensitizer relative to the quantity of silver halide. However, the quantity of dye remaining in the image layer after processing also usually increases and consequently, so does stain.
It is known that the dye elimination capability of photographic materials during aqueous processing is enhanced by water-soluble groups on the dye molecule. Such groups are primarily acid groups, such as the sulfonic acid group or carboxyl group.
EP-A1-04 27 892 describes certain trinuclear merocyanine dyes as sensitizers for the red spectral range to enable good sensitivity with low stain. At least two of the substituents on these dye molecules are supposed to be special water-soluble groups in the form of a free acid, a salt, or in latent form. Examples of such groups are carboxylic and sulfonic acid groups and alkyl and aryl groups substituted with one of these acid groups.
International Patent Application WO 93/02389 describes a silver halide material for rapid processing to ultrahigh contrast and having low stain, based on a chloride-rich emulsion having a hydrazine compound and a contrast-enhancing amino compound. The sensitizer is a benzimidazole carbocyanine having at least one acid-substituted alkyl group on one of the imidazole nitrogen atoms.
U.S. Pat. No. 4,725,532 teaches that pepper is mitigated in high contrast materials having hydrazine compounds and a cationic cyanine, hemicyanine, or rhodacyanine dye, if the materials also contain ascorbic acid.
EP-A1-05 21 632 involves trimethine dyes based on benzothiazole, which are supposed to yield increased sensitivity in the red-sensitive layer of color films and a low tendency to color adulteration from diffusion into another layer.
There is further need for red sensitive materials having high sensitivity and minimal stain, particularly from the standpoint of making silver images by exposing silver halide films with red-emitting light sources, preferably lasers, and subsequent rapid processing.